The present invention relates to optical waveguide filaments, and more particularly to an improved method of forming blanks from which such filaments are drawn.
Optical waveguides, which are the most promising medium for use in optical communication systems operating in the visible or near visible spectra, normally consist of an optical filament having a transparent core surrounded by a transparent cladding material having a refractive index lower than that of the core.
The stringent optical requirements placed on the transmission medium to be employed in optical communications systems has negated the use of conventional glass fiber optics, since attenuation therein due to both scattering and impurity absorption is much too high. Thus, unique methods had to be developed for preparing very high purity glasses in filamentary form. Certain glass making processes, particularly vapor deposition processes, have been commonly employed in the formation of optical waveguide blanks. In one such process, the source material vapour is directed into a heated tube wherein it reacts to form a material which is deposited in successive layers. The combination of deposited glass and tube is collapsed to form a draw blank which can be later heated and drawn into an optical waveguide filament.
In order to obtain uniform deposition along the length of the substrate tube, a serial deposition process has been employed. That is, reactants are fed into the end of the tube, but deposition occurs only in a section of the tube downstream of the region which is heated by a flame. The flame moves up and down the tube to move the reaction and thus the region of glass deposition serially along the tube.
One of the limitations of such a process is a comparatively low effective mass deposition rate. To increase the deposition rate it appears to be necessary to increase the inside diameter of the substrate tube to provide a greater collection surface area. However, since heat is supplied from the outside of the tube, a larger tube diameter results in a lower vapor temperature at the axis of the tube. Moreover, the flow profile across the tube is such that maximum flow occurs axially within the tube. As tube diameter increases, a smaller portion of the reactant vapor flows in that region of the tube adjacent the wall where reaction temperature is higher and where the resultant sooty reaction products are more readily collected downstream of the heated region of the tube.
It is therefore an object of the present invention to improve the deposition efficiency of a process whereby a reactant vapor flows into and reacts within a heated tube to form a layer therein.